x86/bugs: Replace CONFIG_SPECTRE_BHI_{ON,OFF} with CONFIG_MITIGATION_SPECTRE_BHI

For consistency with the other CONFIG_MITIGATION_* options, replace the
CONFIG_SPECTRE_BHI_{ON,OFF} options with a single
CONFIG_MITIGATION_SPECTRE_BHI option.

[ mingo: Fix ]

Signed-off-by: Josh Poimboeuf <jpoimboe@kernel.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: Sean Christopherson <seanjc@google.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Nikolay Borisov <nik.borisov@suse.com>
Link: https://lore.kernel.org/r/3833812ea63e7fdbe36bf8b932e63f70d18e2a2a.1712813475.git.jpoimboe@kernel.org